October 2005
Pepper plant with immature peppercorns. Example of seeds with perisperm are pepper (Piper nigrum) and beet (Beta vulgaris). Source: Koehler’s Medicinal-Plants, 1887.
Plant misconceptions are common in textbooks, science project books, dictionaries, encyclopedias, science education journals, and educational websites. Published misconceptions may misinform large numbers of students and teachers, but when they are known, misconceptions can be useful indicators of concepts that are potentially confusing to students. In an earlier article, I identified 50 published misconceptions that were classified into five categories: oversimplifications, overgeneralizations, obsolete concepts and terms, misidentifications, and flawed research.1 In this article I will present an additional category, undergeneralizations, and 50 more misconceptions about plants.
Undergeneralizations
Undergeneralizations are concepts that are applied too narrowly, such as concepts that are restricted to angiosperms when they also apply to gymnosperms or seedless plants.
Dioecious species are often defined as having staminate (“male” or pollen producing) and carpellate (“female” or ovule bearing) flowers on separate plants. About 40 percent of gymnosperm species are dioecious, with pollen and seeds produced on separate plants.
Monoecious species are often defined as having staminate and carpellate flowers on the same plant. This excludes monoecious gymnosperms that have pollen cones and seed cones on the same plant.
“Perennial” is often used to describe plants that live more than two years and usually flower every year, and yet most nonflowering plants are also perennial.
Texts often imply that only woody plants have secondary growth. Many nonwoody plants have secondary growth; for example, carrot storage roots are mainly secondary tissues.
Obsolete concepts
Texts usually say photorespiration lowers the efficiency of photosynthesis in C3 plants and has no purpose. Recent research reveals that normal nitrate metabolism requires photorespiration.2
The plant hormone, abscisic acid, was misnamed. Ethylene is now considered to be the hormone that triggers abscission.3
Nickel is often omitted as an essential element for plants, yet research published in 1987 showed nickel is essential for plants.4
Florigen was long considered to be merely a hypothetical flowering hormone, but it now appears to be a messenger RNA, according to recent research.5
Phloem is sometimes said to carry organic “wastes,” when the roles of these organic compounds are unknown in plants. Secondary organic compounds are assumed to function in defense if they have no other obvious role.3 Direct evidence for this is often lacking, although it may be forthcoming in future studies. For example, when tobacco was genetically engineered not to produce nicotine, insects attacked plants without nicotine more severely than plants with normal nicotine levels.6
The oldest living seeds were thought to be those from a 1300-year-old lotus (Nelumbo nucifera).7 Recently, carbon-dating of a germinated seed of date palm (Phoenix dactylifera) showed it was 2000 years old.8
A dictionary definition of intelligence can be applied to plants, even though they lack nervous systems, because of their adaptively variable behavior.9-11
Oversimplifications
A science project book claimed chlorophyll and hemoglobin each contain 137 atoms and replacing one atom changes hemoglobin into chlorophyll. That is not possible because chlorophyll a has a molecular weight of under 900 daltons, whereas one subunit of human hemoglobin has a molecular weight of 16,000 daltons. The grain of truth is that chlorophyll and hemoglobin both contain a similar ring structure with a different central ion: magnesium in chlorophyll and iron in hemoglobin.
Stating that gymnosperm means “naked seed” can mislead students because the degree of “nakedness” varies among the four phyla of gymnosperms. Ginkgo seeds are naked from an early stage in their development, but yew seeds are partly surrounded by a fleshy aril, or covering, that is often mistaken for a fruit. Pine seeds are usually hidden by cone scales until maturity, but cones of five Pinus species (Pinus albicaulis and four others) are indehiscent; that is, they remain closed until they decay or are opened by animals. Emphasize that gymnosperms lack flowers and fruits rather than emphasizing their naked seeds.
Cycads are extinct according to one biology dictionary. It’s true that many are endangered, but there are about 100 living species of cycads, including sago palm (Cycas revoluta), a common houseplant.12
Overgeneralizations
Texts often say that seeds require oxygen to germinate. Most do, but there are some seeds, such as Echinochloa oryzicola, that germinate using anaerobic respiration.13
Light is often omitted as a requirement for seed germination, yet some types of seed do require light to germinate, such as the princess tree (Paulownia tomentosa).14
Texts often emphasize the cuticle as an important adaptation for plant survival on land, but that is not true for all plants. Bryophytes that lack a cuticle have adapted to withstand dehydration by rapidly equilibrating with their environment, a mechanism termed poikilohydry.15
Red beets are sometimes said to be colored by anthocyanins. Beets (Beta vulgaris) are among over 11,000 species that have pigments called betalains instead of anthocyanins.16
Perennials are often said to sexually reproduce every year, but many perennials do not. Some sexually reproduce only once, such as bamboo; some may not sexually reproduce at all if they just reproduce asexually.
Misidentifications
The first misconception listed here is an example of a wider problem of botanical terms that have different popular or gardening definitions. Such problem terms include berry, bulb, fertilization, flower, fruit, herb, nut, petal, root, thorn, and variety. A glossary is provided at the end of the article to help illustrate the differences between botanical and popular definitions for these terms (see Author Glossary).
Petunia, tomato, geranium (Pelargonium spp.), wax begonia, coleus, impatiens, snapdragon, and many other nonwoody plants are often called annuals. They are all perennials, but gardeners call them annuals when they are grown for one season before they die from winter cold. Examples of true annuals are corn and zinnia.
One science project book lists celery as a monocot. Celery is a dicot or eudicot. The author probably misidentified the edible celery petiole as a leaf blade and believed it had parallel veins characteristic of monocots.
An educational website misstated that the Amazon water lily (Victoria spp.), with leaves more than 2 meters in diameter, has the largest leaves of any plant. The Guinness Book of World Records lists a tie between raffia palm (Raphia farinifera) and Amazonian bamboo palm (Raphia taedigera), both with leaves up to 20 meters long.
Other misconceptions
Plant reproduction
- Many websites say the plant equivalent of animal parthenogenesis, or reproduction without fertilization, is parthenocarpy, the production of seedless fruit. Parthenocarpy rarely functions in reproduction, however. (Seedless pineapple fruit is an exception because the leafy crown of the fruit can root and form a new plant.) The equivalent of parthenogenesis in plants is apomixis, the production of asexual seeds. Apomictic seeds rarely develop from unfertilized haploid eggs, as in parthenogenesis. Most apomictic seeds develop from diploid eggs that don’t undergo meiosis, or from diploid cells in the surrounding nucellus.17
Asexual reproduction and vegetative reproduction are not synonymous. Vegetative reproduction involves plant parts—stems, leaves, or roots—not involved in sexual reproduction. Apomixis is a method of asexual reproduction but not of vegetative reproduction.
Vegetative propagation does not always produce offspring that are identical to the parent plant. Plants with variegated leaves are chimeras, meaning they have more than one tissue type, green and nongreen. When a variegated snake plant is propagated from leaf cuttings, plants with nonvariegated leaves are produced. The adventitious shoot that develops from the leaf cutting originates from a single cell of one tissue type. To retain the leaf variegation, the snake plant must be propagated by dividing the parent into two or more plants.17
A plant clone is not always produced from a single cell, although it can be. Plant clones are usually grown from shoots, roots, or leaves.
Pollen is not the male gamete, as stated in a science project book. Pollen is the multicellular male gametophyte, which contains male gametes.
An educational website states that endosperm (embryonic food tissue) is located inside cotyledons (embryonic leaves). Cotyledons may digest and absorb the endosperm during development, but it is no longer endosperm after digestion. When present in a mature seed, endosperm always occurs outside the cotyledon(s).
Plant taxonomy
- In a binomial, such as Quercus alba, Quercus is the genus but alba is not the species. Quercus alba is the species, and alba is the specific epithet.
- Some publications italicize cultivar names. Cultivar names are not italicized. They are enclosed by single quotes, for example, ‘Red Delicious’ apple.
- It was once true that plant taxonomists used divisions, which are equivalent to phyla used by animal taxonomists. Plant taxonomists can now use phyla and often do; teachers may want to use phyla for both animals and plants.
Plant anatomy
The term “phloem vessels” appears on many educational websites. This is a misnomer because vessels are structures characteristic of xylem. The correct term is “phloem sieve tubes.”
Drawings of root cross sections often do not show mycorrhizal fungi. This is atypical because about 90 percent of plant species have mycorrhizae, which means “fungus root.”
The food storage tissue of angiosperm seeds is not always endosperm. Some angiosperm seeds have a diploid perisperm as their food storage tissue instead of endosperm. The perisperm develops from ovule cells. Example of seeds with perisperm are pepper (Piper nigrum) and beet (Beta vulgaris).
An educational website states that dead xylem cells, which transport water, are impermeable. Dead xylem cells, both vessel elements and tracheids, could not transport water if they were impermeable.
Plant morphology
- “Staminate cones” is a widely used term, but it is a misnomer because gymnosperm cones lack stamens. The correct term is “pollen cone.”
An encyclopedia states that “suckers” are specialized roots of parasitic plants that steal nutrients from their host plant. The specialized roots of parasitic plants are called “haustoria.”18 Suckers are adventitious shoots that arise from underground roots.
An encyclopedia relates that stems of a strangler fig “strangle” its tree host. Strangler fig (Ficus spp.) usually starts as an epiphyte high in the host tree, and its aerial roots encircle the host trunk and do the “strangling.”19
An educational website claims that dandelion seeds have “parachutes” for wind dispersal, attributing what is often a fruit function to seeds. The dandelion fruit serves as the “parachute.”
Plant physiology
“Tropism” may easily be the most misdefined term in plant biology. Defining tropism as a growth response to an environmental factor is far too vague. Tropism refers to the direction of plant growth being determined by the direction of an environmental factor, such as light, gravity, or touch.
The caption for a photo of a single-rooted philodendron cutting claimed that auxins were applied to promote rooting. Philodendrons have aerial roots, so no auxin application is needed to elongate those preformed roots. (Another generic error is illustrated here: The appropriate control treatment isn’t pictured to make the treatment effect clear.)
A text claimed that a nastic movement, a plant response to certain stimuli, is not caused by an external stimulus. Some nastic movements are caused by an external stimulus, such as rapid closure of a Venus flytrap leaf, but the direction of movement does not depend on the direction of the stimulus, as in a tropism.20
A text stated that the circling of a twining stem before it contacts a support is a nastic movement. That phenomenon is termed circumnutation.21
“Photoperiodism” is often defined as the effect of day length on flowering. This understates the scope of photoperiodism, which also affects seed germination, tuber and bulb formation, leaf abscission, dormancy, adventitious plantlet formation on leaves, and sexual reproduction in some nonflowering plants.22
A text stated that houseplants have little phototropism, which is why they were selected as houseplants. Houseplants do exhibit phototropism and are not selected for their response to light (or lack thereof) but for their ease of production and ability to survive and remain attractive in a home environment.
Plant mineral nutrition
- Squanto is often credited with showing Pilgrims how Native Americans used fish as fertilizer. Squanto did show them this, but it was not a Native American practice where he lived.23 Squanto had been to Europe and learned of fish fertilizer from European farmers. Native Americans practiced shifting agriculture and had no need for fertilizer. They moved on to new land when the soil fertility of a place was depleted.
Texts often say soil pH determines hydrangea flower color. Actually, the aluminum concentration in the flower tissue determines its color.24 Hydrangeas in acidic organic soil form pink flowers because there is often little aluminum in organic soils. Commercial growers who want blue hydrangeas add aluminum sulfate to the soil. This provides sufficient aluminum as well as a low enough pH for the aluminum to be available for plant uptake.
The available form of boron for plants is not H2BO3-. Plants absorb boron mainly as uncharged boric acid, B(OH)23. [It is structurally more accurate to write B(OH)3 than H3BO3.] As a Lewis acid B(OH)3 combines with H2O to produce B(OH)4- and H+.
Sulfates do not lower soil pH. The cation part of the molecule does. Calcium sulfate and potassium sulfate are not used to acidify soil. Aluminum sulfate [Al2(SO4)3] and iron sulfate (FeSO4) are used to acidify soil. Both Al and Fe combine with hydroxyl ions (OH-) to form insoluble hydroxides, Fe(OH)3 and Al(OH)3. Elemental sulfur can acidify soil when Thiobacillus bacteria convert it to sulfuric acid.
The calcium in liming materials does not raise soil pH. The anion part of the liming molecule, such as carbonate or hydroxyl, neutralizes hydrogen ions. Other hydroxides or carbonates, such as magnesium and potassium, also function as liming materials.
Conclusion
Given the frequency of published plant misconceptions, teachers must inform students about the problem. Students and teachers need to maintain a healthy skepticism about plant facts. The bionet.plants.education newsgroup is a good place for teachers to pose questions on suspected plant misconceptions.
© 2005, American Institute of Biological Sciences. Educators have permission to reprint articles for classroom use; other users, please contact editor@actionbioscience.org for reprint permission. See reprint policy.
